1. Field of the Invention
The invention is generally related to the area of optical devices. In particular, the present invention is related to optical wavelength multiplexing/demultiplexer or add/drop devices with new optical layouts and manufacturing processes.
2. The Background of Related Art
Optical add/drop and multiplexer/demultiplexer devices are optical components often used in optical systems and networks. FIG. 1 shows a typical multiplexer/demultiplexer device 100 utilizing what is referred to as a free-space cascading structure. The input signals including wavelengths λ1, λ2, . . . λK, . . . λN is coupled into a common collimator 102 of the device 100. The beam of the input signals is collimated and then propagates in free space before impinging upon a first filter 104. For example, the first filter transmits a wavelength λ1 and reflects all others. As a result, a signal at wavelength λ1 passes the filter 104 and coupled out via a collimator 106.
The reflected signals going through a second filter 108 that transmits a signal at wavelength λ2 and reflects all others. The signal at wavelength λ2 passing through the second filter 108 is coupled out by a collimator 112. The reflected signals from the second filter 108 are successively transmitted and reflected through the remaining filters and collimators. Subsequently, the signals at wavelengths λ1, λ2, . . . λK, . . . λN are all separated through the multiplexer/demultiplexer device 100.
It is, however, noted that the collimators and filters are bonded to a common substrate separately. Thus a filter and a collimator for the same channel (e.g., a particular wavelength) are isolated. The beam angle of incidence (AOI), which influences which wavelength can pass, is adjusted by rotating the filter. For 100 GHz DWDM, the central wavelength control accuracy is required to be within 0.03 nm, which means 0.1° of rotation with fixation accuracy assuming AOI=1.8°. Such accuracy is indeed a challenge for manufacturing such devices in volume. The prior art may work fine for CWDM as the margins of a central wavelength and the bandwidth thereof are relatively more tolerable, but it is difficult to extend such manufacturing process to DWDM devices.
Another significant disadvantage of the prior art devices is that they require coating surface perfectly perpendicular to the substrate. FIG. 2A shows a side view of two filters of FIG. 1, where both filters 116 and 118, or their coating surfaces are perpendicular to the substrate 120, requiring an incoming light beam is reflected back at the same height. If the sidewall of the filter 118 is not perpendicular to the substrate 120 as shown in FIG. 2B, namely its angle Ψ is not 90°, then the reflected beam would point either up or down. Consequently, after multiple bounces between the filters as shown in FIG. 1, the beam may hit the edge of a filter, leading to performance degradation, or hit the substrate, leading to beam clipping loss.
Accordingly, there is a great need for multiplexing/demultiplexer or add/drop devices that can be efficiently manufactured in volume.